This invention relates to constructions of photodetectors, and in particular to those that are sensitive to wavelengths out to 1.6 microns or beyond and are constructed to produce a photocurrent from the absorption of light in In.sub.1-x Ga.sub.x As.sub.y P.sub.1-y, where 0.9.ltoreq.y.ltoreq.1 (i.e. material having a band gap of not more than 0.8 eV).
In moving from GaAs photodetectors through to photodetectors fabricated in smaller band gap material, which are sensitive deeper into the infra-red, the problems of dark current become much more significant. Thus for instance, a photodetector fabricated in n-type GaAs (1.42 eV) by opening a 150 micron diameter window in a silica masking/passivation layer for a zinc diffusion typically provides a device with a dark current of a few tens of picoamps when reverse biassed at 5 volts. In a series of comparative tests, similar structures constructed in InP (1.35 eV) instead of GaAs were found to exhibit a dark current typically of a few nanoamps, structures constructed in InGaAsP (lattice matched with InP) having a band gap of 1.13 eV exhibited dark currents typically of a few tens of nanoamps, but structures constructed in InGaAs (0.75 eV) (also lattice matched with InP) did not show any rectifying characteristics.
The unsatisfactory characteristics of the low band-gap material appear to result from surface effects where the p-n junction is covered by the silica masking/passivation layer. Manufacture of InGaAs photodetectors using silicon nitride instead of silica has been reported. However, the technology required to implement silicon nitride passivation entails certain difficulties not encountered when using silica passivation, and for this reason it is desirable to avoid having to have recourse to silicon nitride technology if at all possible.
British Patent Specification No. 2029639A, to which attention is directed, refers to the problems of leakage current in a photodetector at the boundary of its p-n junction, and discloses how these problems may be reduced or effectively eliminated by an overlay of higher band gap material through which the diffusion is made so that the p-n junction comes to the surface of the semiconductor body in the higher band gap material. Such structures can be made using vapour phase epitaxy to form the overlay, but with liquid phase epitaxy the equilibrium conditions result in severe melt-back problems when attempting for instance to grow a layer of InP upon a layer of InGaAs, and this gives an unsatisfactory interface associated with a disturbed region of growth. This melt-back problem is not confined exclusively to growth on InGaAs, but applies also to growth on InGaAsP having a low phosphorus content, and is for instance still significant when attempting to use liquid phase epitaxy to grow InP on quaternary material having a band gap of 0.85 eV.